1. Field of the Invention
The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with an axially modular fuel assembly and rod for more optimal fuel utilization.
2. Description of the Prior Art
One typical prior art nuclear fuel assembly is formed from a plurality of fuel rods supported in a skeletal assembly composed of top and bottom nozzles, a plurality of guide thimbles and an instrumentation tube extending longitudinally between and connected at their opposite ends to the top and bottom nozzles. A plurality of grids are axially spaced along and mounted to the guide thimbles and instrumentation tube for laterally supporting the fuel rods.
Each fuel rod typically includes an elongated hollow cladding tube and a plurality of uranium dioxide fuel pellets contained in a stack within the tube with the tube sealed at its opposite ends by end plugs Each fuel rod is supported at each grid by a combination of support dimples and springs on the grid. The grids are axially spaced along the guide thimbles and are captured by bulge joints formed in the guide thimbles at the grid locations in order to create an integrated skeletal structure in which the fuel rods can be inserted. The bottom nozzle serves as a lower support structural element for the fuel assembly and directs coolant flow distribution to and through the fuel assembly. The top nozzle functions as the upper structural element of the fuel assembly. The guide thimbles are structural members which also provide channels for guidance of control rods into some of the fuel assemblies.
A conventional nuclear reactor core includes a plurality of the prior art fuel assemblies disposed in a side-by-side array. In such reactor core, the benefits of an extended core life are not fully attained because of incomplete fuel consumption in the axial and radial directions during the life of the core. The incomplete consumption of fuel is most prevalent at the top and the bottom of the core. One approach to alleviation of poor fuel utilization is to employ natural uranium or, seed blankets, at the top and bottom of the core. U.S. Pat. No. 4,493,814 to Beard, Jr. et al and U.S. Pat. No. 4,631,166 to Camden, Jr., both assigned to the assignee of the present invention, relate to this approach.
Another approach is to provide a radially modular fuel assembly composed of a plurality of subassembly fuel modules. U.S. Pat. No. 4,716,015 to Carlson, assigned to the assignee of the present invention, relates to this approach. Each subassembly fuel module includes bottom and top nozzle module sections and grid sections with guide tube thimbles and fuel rods extending between the top and bottom nozzle module sections. Fuel assemblies can be reconstituted using subassembly fuel modules from different fuel assemblies or using one or more new subassembly fuel modules
Each of the above-mentioned approaches fosters some improvement in fuel utilization. However, it is perceived by the inventors herein that optimal fuel utilization has not yet been attained.